In order to binarize an image for output on utilization devices such as bilevel display and printers, or for digital compaction/transmission of such non-coded information, many algorithms are used. These algorithms use a variable threshold determined by an average value of picture elements (pel(s)) close to the picture element for which the current threshold is to be determined. In other words, each individual pel is compared with the neighborhood average of pels which can be obtained by defocusing the image or digitally summing values from surrounding pels. A variety of peturbations have been made on neighborhood threshold techniques, sometimes in response to hardware, or to other limitations. These algorithms have been found to be very useful where edge emphasis is beneficial, for example, in typed documents and line drawings.
Previous devices for generating neighborhood threshold reference levels fall into three categories: (1) devices in which each individual pel in a line is alternately scanned in a focus-defocus fashion by mechanically changing the optical system via a vibrating or rotating mirror, lens, prism, disk or wheel. (2) a flying spot cathode ray tube (CRT) scanner with electronically variable spot size. (3) devices in which data is digitized with an analog-to-digital (A/D) converter with several lines of data being stored, and in which the neighborhood averages are digitally calculated for each pel from the surrounding values of the stored data.
For speeds on the order of 10.sup.6 pels per second, the scheme set forth in (1) is difficult to realize due to the speed of mechanical motion required for each individual pel scan. The CRT scheme in (2) lacks pancromatic response, reliability, linearity of scan, and has deficient defocus at high speeds. The technique in (3) suffers from the problems associated with high speed multi-bit conversion, the expense and speed of digital line buffers and the many additions required for each output decision.
There are a number of patents dealing with the general concept of image scanning. For example, U.S. Pat. No. 4,012,587 to Ochi et al, discloses a solid state image sensor of the type which employs an interline transfer charge-coupled imaging device, in which the electrodes of the vertical shift registers are enlarged to extend to places which will lie between image pick-up portions aligned in a vertical direction of the device. Each of the image pick-up portions comprises an image sensing area and a transfer gate. Preferably, every other image pick-up portion of a conventional CCD imaging device is removed to provide spacing between the image pick-up portions relative to the vertical direction. U.S. Pat. No. 3,932,775 to Kosonocky discloses an interlaced readout of charge stored in a charge coupled image sensing array. Storage of different charges in CCDs in different time periods is disclosed, with internal combination of charges being stored during prior time periods. U.S. Pat. No. 3,993,897 to Burke et al discloses a solid state imaging apparatus comprised of an array of charge storage devices. Readout of the charges stored in a row of devices is accomplished by obtaining the difference of pairs of signals, one sensed on the row line connecting the selected row of devices and the other of the signals sensed on the row line connecting an adjacent row of devices emptied of stored charge. U.S. Pat. No. 3,937,942 to Bromley et al discloses a 2-dimensional, multichannel optical correlation system which employs a light source to illuminate a mask having a plurality of linearly disposed channels, each of which has recording information defined by variations in opacity along its linear length. The light source is modulated as a function of a well known input signal. A multiple element CCD having its elements arrayed in linearly disposed groups along axis parallel to the linear disposed channels of the illuminated mask, is positioned to receive the light energy transmitted by the illuminated mask for developing a charge within each such element commensurate with photo energy received at its discrete position. U.S. Pat. No. 3,940,602 to Lagnado, discloses a 2-dimensional signal processing imager array using charge transfer concepts. The system simultaneously measures the incident optical signal and performs a linear transformation upon the signal. Two CCD registers are arranged on either side of the photo diode array. The difference between the charge in the two registers is taken at the output in order to provide both positive and negative weight for a data pattern that is to be convolved or correlated.
According to the present invention an optical scanner system is disclosed in which a charge transfer device chip is alternately illuminated a line at a time with a focused, then defocused image in order to provide comparison signals for binarization of the image concurrent with providing cancellation of dark current and sensitivity variations. The focusing and defocusing is achieved by interposing an optical path length changing element in the optical path of the scanned line array which is scanned a line at a time rather than an element at a time in a line, thereby providing a lower operation speed for the optical path length changing element.
None of the above-cited art discloses the interposing of an optical path length changing element in the optical path of a scanned line array for the purpose of alternately focusing and defocusing the scanned image. There is no teaching of operating the path length changing element at a reduced speed by scanning a line of elements at a time, instead of a single element at a time in a line. Further, the cited art does not disclose a scanner array which is comprised of a single charge transfer device chip having a simple analog line memory comprised of light responsive elements, shift registers, and gates, and including a comparator means for performing the binarization of the scanned image. Also, there is no suggestion in the above-cited art of how to cancel dark current and sensitivity variations in a scanner system.